This invention relates to improved gear oils, particularly mineral gear oils, and processes for preparing these compositions. In one of its aspects this invention relates to the addition of a small amount of hydrogenated butadiene-styrene copolymer to a gear oil formulation to produce improved formulations that are shear stable and have a high viscosity index (V.I.). In another of its aspects this invention relates to hydrogenated butadiene-styrene copolymers having weight averaged molecular weights within a specified range and also having defined amounts of butadiene and styrene which are blended with suitable mineral oils to produce shear stable gear oils of improved viscosity index.
It is well known from such patents as U.S. Pat. No. 3,554,911 that shear-stable, high viscosity index lubricating oil formulations can be formed by the addition of a small amount of a hydrogenated random butadiene-styrene copolymer having a butadiene content in the range of 30 to 44 weight percent and having a weight average molecular weight in the range of about 25,000 to about 125,000. It has been found, however, that the addition of such copolymers into formulations to be used as gear oils does not produce a gear oil formulation that is shear stable under the temperature and shear conditions met by gear oils in operation. These conditions are sufficiently more severe for gear oils than for crankcase oils that the viscosity of such gear oil formulations rapidly decreases below the acceptable value which has been set at 74 SUS at 210.degree. F (99.degree. C).
It has now, surprisingly, been found that hydrogenated butadiene-styrene copolymers produced similarly to those set out in U.S. Pat. No. 3,554,911, but restricting molecular weight range to less than 20,000 average can be successfully combined into oil formulations to produce shear stable gear oils, that is, oils that retain a viscosity above 74 SUS at 210.degree. F after 50,000 miles of operation. That there is considerable difference in gear oil formulations and lubricating oil formulations can be well appreciated by comparing the fact that a successful gear oil must hold its viscosity characteristics after 50,000 miles of operation while the normal lubricating crankcase oil is expected to be changed after about 2,000 miles of operation.
It is therefore an object of this invention to improve gear oils, particularly the shear stability of mineral gear oils. It is another object of this invention to improve gear oils by adding specific hydrogenated random copolymers thereto. It is still another object of this invention to provide new and improved shear stable gear oil formulations.
Other aspects, objects and the various advantages of this invention will be apparent upon reading the specification and the appended claims.